A study using allozymes as a genetic marker (Eitner 1995) indicated that an unrecognized fourth species of Alopias may exist, however this claim was based on data from a single shark specimen likely to have been A. vulpinus. Recent work investigating population structure of all three Alopias species using mitochondrial DNA control region sequences (Trejo 2004) has found no evidence indicating that a fourth species of thresher sharks exists.

Justification:
All members of genus Alopias, the thresher sharks, are listed as Vulnerable globally because of their declining populations. These downward trends are the result of a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target and bycatch fisheries.

The Common Thresher Shark (Alopias vulpinus) is virtually circumglobal, with a noted tolerance for cold waters. This species is especially vulnerable to fisheries exploitation (target and by-catch) because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries, in which it is readily caught. It is an important economic species in many areas and is valued highly for its meat and large fins. Its life-history characteristics (2-4 pups per litter; 8-14 year generation period) and high value in both target and bycatch fisheries make it vulnerable to rapid depletion. Serious declines have occurred where this species has been heavily fished, for example in the 1980s eastern central Pacific drift gillnet fishery, where reported landings collapsed to 27% of peak levels between 1982 and the late 1980s. Analyses of pelagic longline CPUE data from logbook reports covering the species' entire range in the northwest and western central Atlantic vary according to the time period, but suggest thresher shark stocks declined by between 63-80% during 1986-2000. There is evidence that thresher sharks are being increasingly targeted by pelagic fisheries for swordfish and tuna (e.g., in the Mediterranean Sea) in attempts to sustain catches, and exploitation is increasing in these areas. The high value of the species and its exploitation by unmanaged fisheries combined with its biological vulnerability, indicate that at least some, if not most, subpopulations in other parts of the world are likely to be equally, or more seriously at risk than those for which data are available and, unlike the Californian stock, are not the subject of management, enabling stocks to rebuild.

In addition to the Vulnerable global assessment, a number of regional assessments have also been designated for this species as follows: Near Threatened in the eastern central Pacific; Vulnerable (VU A2bd) in the northwest Atlantic and western central Atlantic; Vulnerable (VU A3bd) in the Mediterranean Sea; and Data Deficient in the Indo-west Pacific.

Eastern Central PacificReported landings in the drift gillnet fishery for this species that developed off the west coast of the USA in the late 1970s, collapsed from a peak of 1,089.5 t in 1982 to less than 300 t by the late 1980s (decline of ~70%). This fishery was effectively eliminated by restrictions on the use of gill nets by 1990, and the population began to slowly recover to just below 50% of the initial subpopulation size. The Common Thresher Shark is still caught as bycatch or as a secondary target, although to a far lesser extent, of the swordfish gillnet fishery. It is clear that the species depends on adequate management measures, and would otherwise be at risk of overfishing. All this considered, the species is assessed as Near Threatened in this region based on significant population declines, which are now managed in US waters.

Northwest and Western Central AtlanticEstimates of trends in abundance from standardized catch rate indices of the U.S. pelagic longline fishery suggest that this species has likely undergone a decline in abundance in this region. Thresher sharks are generally recorded by genus by observers as well as in logbooks, which includes both Common Thresher Shark and Bigeye Thresher Shark (A. superciliosus) in this region, of which Common Thresher Shark is the less common. The area covered by the analyses, ranging from the equator to about 50°N, encompasses the confirmed range of threshers in this region. Estimates of the decline based on logbook and observer records of combined thresher sharks from 1986-2005 range between 50-80%. Fishing pressure on thresher sharks began over two decades prior to the start of this time series, thus the estimated declines are not from virgin biomass. Furthermore the sample size in the latter observer analysis was also very small compared with the logbook analyses which both showed declines. Given the apparent decline in abundance in this region and high fishing pressure from pelagic fleets, this species is assessed as Vulnerable A2bd in the northwest and western central Atlantic.

Mediterranean SeaAdults and juveniles of Common Thresher Shark are regularly caught as bycatch in longline, purse seine and mid-water fisheries throughout the Mediterranean Sea, as well as in recreational fisheries. The species has some important parturition and nursery areas this region, for example the Alboran sea, where aggregations of pregnant females have been observed. Recent investigations show that pelagic sharks, including this species, are being increasingly targeted in the Alboran Sea by the Moroccan swordfish driftnet fleet. Data from this fishery suggest that both annual catches and mean weights of Common Thresher Shark have fallen as a result of fishing mortality. Given that pelagic fishing pressure is high and ongoing throughout the Mediterranean Sea, increased targeting and the decline in catches described above, Common Thresher Shark is currently assessed as Vulnerable (VU A3bd) in this region.

Northeast AtlanticThe Common Thresher Shark is taken as primarily as bycatch of longline fisheries for tuna and swordfish in the Northeast Atlantic, and also in driftnets and gillnets. It is very likely that this catch is retained. Limited information is available on thresher shark catch in this region and estimated landings are still considered incomplete. Prior to 2000, estimated landings fluctuated at 17-13 t, in 2000-2001 they exceeded 100 t, after which they dropped to 4 t in 2002 and have not exceeded 7 t since. Increased targeting of pelagic sharks by Moroccan drift-netters in the Alboran Sea and Strait of Gibraltar mentioned above, has also likely impacted Common Thresher Shark in the northeast and eastern central Atlantic. The species is currently assessed as Near Threatened in this region and there is a need to collect further data on the status the species in this area.

Indo-West PacificLittle information is currently available on Common Thresher Shark in the Indo-West Pacific. Whereas records of Bigeye Thresher Shark and Pelagic Thresher are recorded in the catches of fisheries operating in this region, albeit very under-reported, very little information is available on catches of Common Thresher Shark. Although pelagic fishing effort in this region is high, with reported increases in recent years, the Common Thresher Shark is more characteristic of cooler waters and further information needs to be collected on records and catches of the species in this region.

This oceanic and coastal shark is virtually circumglobal in tropical to cold-temperate seas, but is most common in temperate waters (Compagno 2001).

Western Atlantic: ranges from Canada (from Bay of Chaleur, Gulf of St. Lawrence, south), USA (whole Atlantic Coast, although it is rare south of New England and in the Gulf of Mexico), Cuba, Mexico, Venezuela, south to Argentina (Bigelow and Schroeder 1948, Russell 1993, Anonymous 1997, Compagno 2001).

Eastern Atlantic: ranges from Norway and the UK, south, including the Mediterranean and Black Seas, Madeira, the Azores, and down the coast of western Africa (Ghana, Côte d'Ivoire) to Angola, Namibia, and South Africa (Moreno et al. 1989, Compagno 2001).

Eastern Pacific: From British Columbia south, along the western coast of the USA (Washington, Oregon, California), Mexico, south to Panama, and from Colombia to southern Chile (Compagno 2001). Also reported off the Hawaiian Islands (Compagno 2001).

Common Thresher Sharks appear to exhibit little to no immigration and emigration between geographic areas. The California drift gill net fishery provided insight into the population dynamics of this species, indicating that numerous isolated subpopulations or stocks exist globally (see Threats section). An ongoing global population genetic study (Trejo 2004) using DNA sequences from the mitochondrial control region indicates significant structuring of A. vulpinus populations within the Pacific Ocean, and significant structure between Pacific and northwest Atlantic populations. Differences in length at maturity and fecundity of specimens examined from different regions of the world also provides evidence for isolated subpopulations or stocks (Gubanov 1972, Moreno et al. 1989, Bedford 1992). In the absence of records of transatlantic migrations a single northeast Atlantic and Mediterranean stock of A. vulpinus is assumed (ICES 2007).

The Common Thresher Shark is virtually circumglobal in temperate oceans, penetrates into tropical waters (Compagno 1984), and also has a noted tolerance for cold waters (Castro 1983, Moreno et al.1989). While found both in coastal and oceanic waters, it is most abundant in waters up to 40 or 50 miles offshore (Strasburg 1958, Gubanov 1972, Moreno et al. 1989, Bedford 1992). It ranges between surface waters and 366 m depth (Compagno 1984).

Maximum recorded size varies with sex and geographic location and ranges from 415-573 cm total length (TL) (Gubanov 1972, Cailliet et al. 1983, Compagno 1984, Moreno et al. 1989). Size at maturity varies. Females are reported to mature from 260-465 cm total length (Strasburg 1958, Cailliet et al. 1983, Bedford 1983, Bigelow and Schroeder 1948, Gubanov 1978, Ebert 2003) and males at 260-426.7 cm TL (Cailliet and Bedford 1983, Cailliet et al. 1983). Estimated age at maturity for females ranges from 3-9 years (Bigelow and Schroeder 1948, Strasburg 1958, Gubanov 1978, Cailliet et al. 1983, Cailliet and Bedford 1983, Hanan 1984) and from 3-7 years for males (Cailliet et al. 1983, Cailliet and Bedford 1983). The species reaches an age of at least 24 years (Gervelis 2005) and Cailliet et al. (1983) stated that off California it may reach an age of 50 years; providing k coefficients from the von Bertalanffy growth equation ranging from 0.158-0.215. These parameters were early estimates based on a sample size of 143 specimens (16 male, 23 female, 104 unknown). The most recent estimates of generation time are 8-14 years (Cortés 2008 unpubl. data).

Alopias vulpinus is viviparous, with oophagy and a gestation period of nine months (Gubanov 1978, Moreno et al. 1989, Bedford 1992, Gilmore 1993, Cailliet et al. 1983). Litter sizes range from only two in the Indian Ocean to between 3-7 in the Northeast Atlantic, while 3-4 (predominantly four) pups are common in the eastern Pacific (Gubanov 1972, Gubanov 1978, Holts 1988, Cailliet and Bedford 1983, Hanan 1984, Moreno et al. 1989). Size at birth is 100-158 cm TL (Cailliet et al. 1983, Hanan 1984). Ratios of male to female pups also vary geographically. Moreno et al. (1989) noted a high degree of sexual and size segregation in the northeast Atlantic during pupping season. Mating occurs in middle to late summer and parturition occurs during the spring in both the Northeast Atlantic and the eastern Pacific (Moreno et al. 1989, Bedford 1992). In the Indian Ocean, there is a high degree of sexual segregation, between January and May, with pregnant females in the western Indian Ocean and males around the Maldives (Gubanov 1972). However, pregnant females have also been noted in August and November indicating that birth of young thresher sharks in this area occurs throughout the year (Gubanov 1978). Size at parturition varies considerably, from 115 cm and 156 cm TL with slight variation among geographical locations (Compagno 1984, Moreno et al. 1989). Young A. vulpinus, in all locations, generally remain close to shore after parturition and during their first few years (Moreno et al. 1989).

The meat is highly prized fresh for human consumption and is also eaten smoked and dried salted; the fins are valuable for shark-fin soup; the hide is usable for leather and the liver oil can be processed for vitamins (Compagno 2001). Thresher shark is one of the most important and prized species in recreational fisheries.

Members of the genus Alopias, thresher sharks, are threatened from a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target (for fins and their valuable meat) and bycatch fisheries.

Many countries fish thresher sharks commercially throughout their extensive ranges, with A. vulpinus probably being the most important species (Compagno 1990). This species is frequently caught by offshore longline and pelagic gillnet fisheries (Maguire et al. 2006), is also fished with anchored bottom and surface gillnets, and is a bycatch of other gear including bottom trawls and fish traps (Maguire et al. 2006). Although sometimes referred to as a bycatch in fisheries for other pelagic fishes, this bycatch is normally utilised and would better be described as a secondary target catch. They are also an important sport fishery resource, the meat is considered excellent for consumption, and the large fins highly valued. The growing and largely unregulated shark fin trade also represents a serious threat to thresher sharks. Clarke et al. (2006) report that thresher sharks compose at least 2-6% of the trade in a market study using DNA-based species identification techniques.

The impact of fisheries on A. vulpinus on a global scale, while difficult to assess, has most likely been significant. For example, Japanese and Russian vessels fish the northwest Indian Ocean and central Pacific, and Mexican-Japanese joint ventures have operated longline vessels off Baja California, Mexico, for many years. Furthermore, the number of pelagic sharks landed by fishing fleets in all oceans has become increasingly important in recent years (Mejuto et al. 2006). However, catch statistics are not available (Compagno 2001, Holts 1988, Smith 1998) and where they are, they are under-reported. A recent FAO review of the status of highly migratory pelagic species states: "unless demonstrated otherwise, it is prudent to consider Alopias species as being fully exploited or overexploited globally" (Maguire et al. 2006).

Eastern Central PacificA target pelagic gillnet fishery for this species developed off the west coast of the USA, Eastern Central Pacific (particularly California, and also Washington and Oregon) in the late 1970s (Goldman 2005, Maguire et al. 2006). This fishery serves as a well documented case of population depletion and provides strong evidence that there are numerous isolated subpopulations or stocks globally. Starting with 15 vessels in 1977, the fishery expanded to over 225 vessels in 1982 (Holts 1988, Hanan et al. 1993). The fishery peaked in 1982 with reported landings of 1,089.5 t (Anonymous 1993), declining due to overfishing to less than 300 t by the late 1980s (Maguire et al. 2006). Fishing had heavily reduced the number of juvenile and subadult A. vulpinus off central and southern California, virtually eliminating them from the catch. In 1996, California catches of Common Thresher Shark were down to one-fifth of former levels (Smith 1998). This fishery was effectively eliminated by restrictions on the use of gill nets by 1990 (Bedford 1992, Smith 1998, Maguire et al. 2006). It was originally believed that a Pacific-wide distribution of the species would act as a buffer against over-harvesting (Bedford 1992, Smith 1998). However, this was shown not to be the case, as that portion of the population remained at low levels for several years, and is only reappeared in the catch records and in market places some time later (Bedford 1992, Smith 1998). The species is still caught as bycatch or as a secondary target, although to a far lesser extent, of the swordfish gillnet fishery and may be sold for higher prices in the market than swordfish (Bedford 1992, Smith 1998, Maguire et al. 2006).

While the majority of fishery data for this species in the Pacific has come from California, it is fished in numerous locations throughout its range there. The Spanish pelagic longline fishery for swordfish and sharks is expanding rapidly in the Pacific, with effort expanding from the traditional grounds in the southeast Pacific into the central south Pacific and areas of the north Pacific in recent years (Mejuto 2005). A. vulpinus is taken in both artisanal and commercial longline fisheries in areas off South America, including Peru and Chile (M. Romero pers. comm., Bonfil et al. 2005). Hong Kong customs data shows that Peru is amongst the 20 countries that export the most dried fins to Hong Kong (Bonfil et al. 2005).

Indo-West PacificLittle information is currently available on A. vulpinus in the Indo-West Pacific. Whereas records of A. superciliosus and A. pelagicus are recorded in the catches of fisheries operating in this region, albeit very under-reported, very little information is available on catches of A. vulpinus. Although pelagic fishing effort in this region is high, with reported increases in recent years, A. vulpinus is more characteristic of cooler waters and further information needs to be collected on records and catches of the species in this region.

Northwest and Western Central AtlanticThresher sharks are also an important pelagic species in the north Atlantic, although A. vulpinus is only sporadically recorded in the northwest and western central Atlantic and A. superciliosus is the more common thresher shark in this area. Thresher sharks are generally recorded by genus by observers as well as in logbooks. Analysis of U.S. Atlantic pelagic longline data from their scientific observer program for 1992-2005, suggest that the region of the US east coast where A. vulpinus is somewhat common is from about 35°N-40°N (approximately North Carolina to Philadelphia), where A. vulpinus: A. superciliosus are caught in a ratio of ~1:3 (Baum unpublished data). The first longline fisheries in the Atlantic were begun by the Japanese in 1956 in the western equatorial waters (Uozumi and Nakano 1996). The fleet expanded rapidly in the 1960s, and covered almost the entire Atlantic by the late 1960s (Bonfil 1994), including the areas currently fished by the American fleet. Fishing pressure is high and ongoing and as A. vulpinus is an incidental catch in these fisheries monitoring of catches of this species is extremely limited. Currently, there are no management measures specific to this species in any EEZ or within international waters, and no stock assessments have been done. In Canada and the U.S., less than 5% of the pelagic longlining fleets are monitored by observers, making it difficult to elucidate reliable trends in abundance from these data. Fisheries monitoring in international waters is even more limited. The pelagic longline fishing grounds for the US fleet extend from the Grand Banks (about 45°N) in the northwest Atlantic to 5-10°S off the South American coast, within which geographical areas of longline fishing are defined for classification (Cortés et al. 2007).

Baum et al. (2003) concluded from their analysis of Northwest Atlantic pelagic longline data that the relative abundance of all thresher sharks (A. vulpinus and A. superciliosus combined) had declined 80% from 1986-2000. This analysis is based on estimates of trends in abundance from standardized catch rate indices of the U.S. pelagic longline fishery logbook data, and the fifteen year time period is over one generation length for this species. Although the analysis is not species specific, the sample size of thresher sharks in this data is over 20,000. Furthermore, the area covered by the dataset analysed, ranging from the equator to about 50°N, encompasses the confirmed range of A. vulpinus in these two regions (Compagno 2001). An alternative analysis of the same logbook dataset for 1986-2005 that also combined A. vulpinus and A. superciliosus, resulted in an overall decline of 63% (Cortés et al. 2007). Fishing pressure on thresher sharks began over two decades prior to the start of this time series, thus the estimated declines are not from virgin biomass.

A more recent analysis of Alopias species trends from scientific observer data between 1992 and 2005 in the same U.S. pelagic longline fishery found an almost identical instantaneous rate of decline (-0.12 up to the year 2000) as in the logbook analysis (Baum et al. unpublished manuscript). For this nine year period (1992-2000), the decline amounts to 68%, therefore the decline back to when the fishery started in the 1960s (less than three generation period of 51 years) would be much greater. However, because of recent increases in the catch rates in 2004 and 2005, the overall trend from 1992-2005 of -0.024 was non-significant, and would amount to only a 26% decline (Baum et al. unpublished manuscript). Cortés et al. (2007) also conducted an alternative analysis of this same observer dataset for the same time period that also combined A. vulpinus and A. superciliosus. This analysis of the observer dataset showed a trend opposite to that of the logbook analysis, with a 28% increase since 1992. In contrast, the nominal observer series showed a 39% decline and the logbook index for the same time period showed a decrease of 50%. Furthermore the sample size in the observer analysis was much smaller (n=14-84) than that in the logbook analysis (n=112-1,292) and thus the trend estimated should be regarded with caution. Cortés et al.'s (2007) observer analysis was restricted to four out of the 11 geographical areas covered by the pelagic longline fishing fleet to keep a balanced statistical design (Cortés et al. 2007). Their full logbook analysis, which showed an overall decline of 63%, had much larger sample sizes and is thus better to estimate trends with more certainty (Cortés et al. 2007).

Southwest AtlanticThe thresher shark A. vulpinus is not as common in regional longline catches compared as the bigeye thresher A. superciliosus. Amorim et al. (1998) document its occurrence in the Santos (São Paulo) tuna longline fishery as "low" with only six specimens observed from 1974 to 1996. Gadig et al. (2001) reported on small numbers of juveniles taken by gillnet off São Paulo State.

Mediterranean SeaAdults and juveniles of Alopias vulpinus are regularly caught as bycatch in longline, purse seine and mid-water fisheries throughout the Mediterranean Sea, as well as in recreational fisheries (Lipej et al. 2004). This species has some important parturition and nursery areas in the Mediterranean (Adriatic and Alboran Seas). Moreno and Moron (1992) observed aggregations of pregnant females of A. vulpinus in the Strait of Gibraltar.

Even though driftnetting is banned in Mediterranean waters, this practise has continued illegally (WWF 2005). The Moroccan swordfish driftnet fleet in the Alboran Sea operates year round, resulting in high annual effort levels (Tudela et al. 2005). Even though sharks are a secondary target or bycatch of this fishery, some boats deploy driftnets 1-2 miles from the coast where the chance of capturing pelagic sharks is higher. The catch rate for A. vulpinus is higher in boats actively fishing for sharks (from 0.7 to 1.5 N/fishing operation and 0.09 to 0.11 catch per km net). Both annual catches and mean weights of Alopias vulpinus have fallen as a result of fishing mortality in the Moroccan driftnet fishery, illustrating the likely impact of this illegal fishery on stocks in the Alboran Sea and adjacent Atlantic (Tudela et al. 2005). Valeiras et al. (2003) also report that pelagic sharks are forming an increasing proportion of the catch of Spanish swordfish sleets. Pelagic fishing pressure is high and ongoing throughout the Mediterranean Sea (Tudela 2004, Megalofonou et al. 2000).

Northeast AtlanticA. vulpinus is caught primarily as a bycatch of longline fisheries for tuna and swordfish in the northeast Atlantic, and are also taken in driftnets and gillnets (ICES 2005, 2007). As a highly valuable species, it is very likely that this bycatch is retained (ICES 2005). Limited information is available on thresher shark catch in this region. ICES 2006 reports estimated landings of thresher shark at 13-107 t from 1996 to 2005 in the ICES area, however these data are still considered incomplete. Prior to 2000, estimated landings fluctuated at 17-13 t, in 2000-2001 they exceeded 100 t, after which they dropped to 4 t in 2002 and have not exceeded 7 t since. Increased targeting of pelagic sharks by Moroccan drift-netters in the Alboran Sea and Strait of Gibraltar (Tudela et al. 2005), mentioned above, has also likely impacted A. vulpinus in this area.

Time and area restrictions were imposed off California, USA, since 1990, but A. vulpinus is not managed internationally and there are no quotas.

Family Alopiidae is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea, which urges States to cooperate over the management of these species. No such management yet exists. Precautionary adaptive collaborative management of target and bycatch fisheries is urgently needed for this biologically vulnerable shark. It is also essential to improve data collection and develop stock assessments for this species. Listing on international resource management agreements, such as the Convention on Migratory Species (CMS) could help to drive improvements in national and regional management and facilitate collaboration between states, for this species and other migratory sharks.

The adoption of shark finning bans by fishing states (e.g., USA, Australia), regional entities (EU) and regional fisheries organisations (e.g., ICCAT, IATTC, WCPFC) is accelerating and should increasingly prevent the fishing of thresher sharks for their fins alone.

Compagno, L.J.V. 2001. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Volume 2. Bullhead, Mackerel and Carpet Sharks (Heterodontiformes, Lamniformes and Orectolobiformes). FAO, Rome.

Cortés, E., Brown, C. and Beerkircher, L.R. 2007. Relative abundance of pelagic sharks in the Western North Atlantic ocean, including the Gulf of Mexico and Caribbean Sea. Gulf and Caribbean Research 19(2): 37-52.

Megalofonou, P., Damalas, D., Yannopoulos, C., De Metri, G., Deforio, M., De La Serna, J.M. and Macias, D. 2000. By catches and discards of sharks in the large pelagic fisheries in the Mediterranean Sea. Comm. Of the European Communities.

Mejuto, J., García-Cortés, B. and Ramos-Cartelle, A. 2006. An overview of research activities on Swordfish (Xiphias gladius) and the by-catch species, caught by the Spanish longline fleet in the Indian Ocean. IOTC 2006-WPB-11.

Moreno, J.A. and Morón, J. 1992. Comparative study of the genus Isurus (Rafinesque, 1810) and description of a form ('marrajo criollo') apparently endemic to the Azores. Australian Journal of Marine and Freshwater Research 43: 109-22.

Tudela, S. 2004. Ecosystem effects of fishing in the Mediterranean: an analysis of the major threats of fishing gear and practices to biodiversity and marine habitats. Studies and reviews. No. 74. General Fisheries Commission for the Mediterranean, Food and Agriculture Organization. 44pp, Rome.

Uozumi, Y. and Nakano, H. 1996. A historical review of Japanese longline fishery and billfish catches in the Atlantic Ocean. Collective volume of scientific papers. Report of the second ICCAT Billfish Workshop. International Commission for the Conservation of Atlantic Tunas, Madrid..